Medical, nursing and veterinary care often involves procedures where instruments are introduced blindly into the human or animal body. X-ray and fibre-optic imaging may be used to guide certain high risk procedures, but the vast majority of routine procedures are conducted “blind” such that the practitioner cannot see where the instrument is going nor clearly identify its final location.
Catheters are tubes, which typically drain fluid from, or introduce fluid into, the body. Catheters have been available since the eighteenth century, and their use is now common practice in medicine. They are used extensively with babies in neonatal units and catheters may be inserted, for example, via the nose or mouth into the stomach or jejunum for feeding. Nasogastric or nasojejunal feeding is the preferred method for providing nutritional support to critically ill patients of all ages. Feeding catheters may also be inserted directly into the stomach through an incision in the abdominal wall (known as gastrostomy tubes).
There are two main categories of catheters which are currently used for providing nutritional support. One includes firm, large bore, plastic tubes which are typically used for short term feeding. The other category includes softer, narrow bore, polyurethane tubes, which show less side effects such as ulceration and bleeding of the nose, pharynx and stomach than the firm tubes, but which are not without risks themselves. In particular, as they require a stylet for insertion, tracheopulmonary injuries associated with insertion of these tubes have been reported (Rubenoff R and Ravich WJ, Pneumothorax due to nasogastric feeding tubes, Archives of Internal Medicine 149 184-188, (1989)).
Whichever type of catheter is being used, and regardless of the age of the patient, the key issue for the practitioner is verification of the correct placement of the catheter. This must be done when the catheter is initially passed and, for example, before every feed or administration of medication. Particular problems can arise with specific groups of patients such as neonates or critically ill patients who are paralysed and ventilated.
It is often necessary, particularly in pediatrics, to teach non-medical persons (e.g. parents) how to use catheters to administer enteral feeds and medication, so that they may deliver care either under supervision in hospital, or at home where there is no direct supervision or easy access to nursing support.
A feeding catheter may be considered to be misplaced if it is placed in the respiratory tract, oesophagus, or intestine if gastric feeding is desired, or in the stomach if intestinal feeding is the intention. Whilst serious complications and death are rarely reported, malpositioned catheters can cause problems for patients, such as intolerance of feeds. Having a catheter passed is unpleasant for both adults and children, and having to have this process repeated because the catheter is in the wrong place, or because the location cannot be verified, is costly in both time and distress to the patient. Those administering the tube, be they nurses, doctors or parents/carers, the fear of pouring feed into the lungs creates considerable anxiety and is not conducive to promoting confidence.
A variety of methods have been, or are currently, used to verify the position of feeding catheters. However, published research studies that have been conducted have found such clinical indicators to be unreliable. (Metheny et al., Nutrition in Clinical Practice, 19(5) 487-495 (2004); Rassias et al., Clinical Care 2 (1) 25-28 (1998); Rubenoff and Ravich, loc. cit.; Bohnker, Artman and Hoskins, Nutritional Clinical Practice. 2 203-209 (1987)). The most common methods for verifying catheter placement including the following:
Auscultation involves injecting air into the catheter and listening for a loud sibilance as this air enters the stomach. This procedure is not only unreliable, but may also be dangerous if the tube is misplaced. For example, if the tube is wrongly placed in the lungs and air is injected directly into the pulmonary parenchyma, it is possible to induce a pneumothorax (Rubenoff and Ravich, loc. cit.). In neonates, the stomach and lungs are so close together that it is virtually impossible to distinguish between the sound of air being injected into the stomach and air injected into the bronchi. Even in adult patients the sound of injected air can be heard over the epigastric region regardless of the location of the tube.
Observing patients for respiratory distress (such as coughing, choking, dysphagia or the inability to speak) is also an unreliable indicator of tube misplacement. Small bore tubes can enter the respiratory tract with few, if any, symptoms (Metheny et al Heart Lung Journal of Acute Critical Care 19, 631-638 (1990)) and large bore tubes can enter the respiratory tract with no symptoms particularly if the patient is unconscious (Rassias et al., loc. cit.). Observing for bubbling when the end of the tube is placed in water is unreliable as the stomach may also contain air and produce bubbling.
Testing aspirate of the tube, for pH for example, has been used for some years. Studies have reported that, excluding radiographic confirmation, a pH reading of 4 or less is the most reliable indicator of tube placement in the stomach (Metheny et al., loc. cit., Metheny and Titler, Am. Journal of Nursing 101 36-45, (2001)). However, whilst it is claimed that the pH testing of aspirate is a simple and easy test in the clinical setting (Westhus, Am. Journal Maternal Child Nursing 29 282-291, (2004)), there are problems with this method. Neonates in particular pose specific considerations as they have an intrinsically higher gastric pH. Medications and continuous feeding may also affect the gastric pH, and intestinal and pulmonary aspirate has a pH greater than 5 so this method could not be used to differentiate between these two locations. The pH of aspirate is also unable to act as a means to differentiate between oesophageal and gastric placement as oesophageal reflux can result in both acidic and alkaline readings.
Laboratory tests for gastrointestinal enzymes, pepsin and trypsin are more accurate predictors of placement (Gharpure et al., Critical Care Medicine 25 2962-2966, (2000)) but at present there are no simple bedside tests for these enzymes. In addition, like determination of pH value, such tests would require the aspiration of fluid from the stomach.
Very few of the research studies on assessment of aspirate address the difficulty that can occur when trying to obtain aspirate from patients. This is particularly a problem in neonates and when using fine bore tubes. The National Patient Safety Agency offers advice on how to address the problems of obtaining aspirate from neonates which nurses should follow. However, anecdotal evidence suggests that nurses may use syringes of an inappropriate size to obtain aspirate which may damage the gastric mucosa of the baby or cause the catheter to collapse, both of which are potentially harmful and at least require the tubes to be removed and a new one to be inserted. Tubes with built in pH probes that do not rely on obtaining aspirate have been developed and used clinically. However, they have been mainly used to distinguish between placement in the stomach and small intestine rather than between gastric and pulmonary placement (Berry, Shoettker and Orr, Nutrition 10 419-423 (1994)). Such tubes are expensive and require special training to be used.
X-ray examination has long been considered the only definitive determination of the placement of catheters. Thus, catheters generally are either radiopaque or have a radiopaque strip so that they may be seen via X-ray examination. However, interpretation of X-rays, particularly in the critically ill can be difficult and errors in interpretation can occur resulting in significant morbidity (Rassias et al., loc. cit.). Exposure to radiation also carries significant long term risks, particularly in neonates.
Thus, there is no current, definitive, non-radiographic method to determine correct placement of a catheter, for example by differentiating between respiratory, oesophageal, gastric and small bowel placement.
The present invention therefore aims to overcome, or ameliorate, these problems in the prior art, and provide products and methods for determining correct placement of a catheter within a bodily passage or cavity.